Piezo-diode cantilever MEMS

ABSTRACT

A piezo thin-film diode (piezo-diode) cantilever microelectromechanical system (MEMS) and associated fabrication processes are provided. The method deposits thin-films overlying a substrate. The substrate can be made of glass, polymer, quartz, metal foil, Si, sapphire, ceramic, or compound semiconductor materials. Amorphous silicon (a-Si), polycrystalline Si (poly-Si), oxides, a-SiGe, poly-SiGe, metals, metal-containing compounds, nitrides, polymers, ceramic films, magnetic films, and compound semiconductor materials are some examples of thin-film materials. A cantilever beam is formed from the thin-films, and a diode is embedded with the cantilever beam. The diode is made from a thin-film shared in common with the cantilever beam. The shared thin-film may a film overlying a cantilever beam top surface, a thin-film overlying a cantilever beam bottom surface, or a thin-film embedded within the cantilever beam.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to the fabrication ofmicroelectromechanical systems (MEMS) and, more particularly, to athin-film diode cantilever MEMS and related fabrication procedures.

2. Description of the Related Art

Active devices such as thin-film transistors (TFTs) and diodes areformed through deposition processes that create thin films of silicon(Si) and insulator material. While the resulting devices may not havethe switching speed and drive capability of devices formed onsingle-crystal substrates, they can be fabricated cheaply with arelatively few number of process steps. Further, thin-film depositionprocesses permit active devices to be formed on alternate substratematerials, such as transparent glass substrates, for use in liquidcrystal displays (LCDs). More specifically, the active devices mayinclude a deposited amorphous Si (a-Si) layer. To improve theperformance of the device, the a-Si may be crystallized to formpolysilicon, at the cost of some extra processing. The crystallizationprocedures are also limited by the temperature sensitivity of thesubstrate material. For example, glass substrates are known to degradeat temperatures over 650 degrees C. Large scaled devices, integratedcircuits, and panel displays are conventionally made using thin-filmdeposition processes.

MEMS devices are a logical derivative of semiconductor IC processes thatmay be used to develop micrometer scale structural devices such astransducers or actuators. MEMS devices interface physical variables andelectronic signal circuits. MEMS structures are varied and, therefore,more difficult to standardize, as compared to the above-mentioned thinfilm processes. On the other hand, it may be possible to develop MEMSdevices by engineering modifications to well-developed silicon ICprocesses. Many of the MEMS devices that have been fabricated to datehave more theoretical than practical application, as the devices areoften difficult and expensive to make. For the same reason, larger scalesystems using MEMS components have been expensive to fabricate due tothe process difficulties and the cost associated with integrating theMEMS and IC technologies.

For example, transistors and associated MEMS structures have beenfabricated on bulk Si substrates, and the authors claim excellentperforming biochemical sensing MEMS transducers [Vinayak P Dravid andGajendra S Shekhawat; “MOSFET Integrated Microcantilevers for NovelElectronic Detection of “On-Chip” Molecular Interactions”, MaterialScience, Northwestern University, Evanston, Ill.]. However, the etchingprocesses needed to form a bulk silicon MEMS are more difficult tocontrol, dramatically limit available process steps, and require longetch times. These limitations make these devices unsuitable for low-costintegrated systems.

Alternately, MEMS structures made using high temperature LPCVD thinfilms have been built with conventional sensing schemes such ascapacitive and/or piezoresistive bridges, generating reasonable outputsignals [(1) William P. Eaton, James H. Smith, David J. Monk, GaryO'Brien, and Todd F. Miller, “Comparison of Bulk- andSurface-Micromachined Pressure Sensors”, Micromahined Devices andComponents, Proc. SPIE, Vol. 3514, P. 431. (2) Joao Gaspar, Haohua Li,Paulo Peieiro Freitas, “Integrated Magnetic Sensing ofElectro-statically Actuated Thin-Film Microbridges”, Journal ofMicroElectroMechanical Systems, Vol. 12, No. 5, October. 2003, p.550-556]. However, these sensing schemes cannot be applied to lowtemperature TFT process, because the changes in electricalcharacteristics induced as a result of stress change are too small to bepractically measured.

Stress is induced on a surface when bio-molecules become immobilized ona solid surface. This property is one of the most promising avenues toexplore for bio-sensing. To detect the surface stress, a thin cantilevermay be used. The selective absorption or immobilization of molecules onone side of the cantilever creates a surface stress difference betweenthe two sides of the cantilever, and this difference is measured via achange in electrical resistance using an integrated piezoresistortransducer. Alternately, the substantial displacement at the cantilevertip can be detected by an atomic force microscope (AFM). AFM has thebest sensitivity, but its expense and complexity prevent it from beingwidely used. Some key design issues include strain sensitivity and theelectrical noise inherent in the sensor. These problems areconventionally addressed by using a single crystal silicon substrate.

Electrically passive piezoresistive cantilever transducers have beenstudied and demonstrate bio-sensing capabilities for low surface stresssensing. Limited by single crystal silicon anisotropic fabricationprocesses and relatively poor sensitivity, it is difficult to fabricatea piezoresistive cantilever sensor array at low cost. Prior art devicesare usually formed on silicon-on-insulator (SOI) wafers, using hightemperature processes and special tools such as deep reactive ionetching (RIE). Bulk micromachining uses a subtractive process to carvethe MEMS structure out of the bulk substrate (typically a siliconwafer).

It would be advantageous if a high sensitivity MEMS cantilever could beformed with an integrated active device from laser annealed thin-films,without the necessity of a single crystal silicon substrate or bulkmicromachining processes.

SUMMARY OF THE INVENTION

Sensitivity and signal-to-noise ratio are two important parameters forMEMS sensors. The present invention piezo-diode cantilever MEMS sensorconverts mechanical energy (surface stress) induced on a molecule-solidinterface to electrical energy. Reducing the MEMS structure thicknesseffectively enhances the mechanical strain induced by surface stress. Inone aspect, the present invention uses a plasma enhanced chemical vapordeposition (PECVD) method to control the MEMS structure thickness, anduses a pre-deposited sacrificial film to support the MEMS structurefabrication and define the air gap between a MEMS structure and thesubstrate. The sacrificial film is removed to free the MEMS structureafter all fabrication steps are completed. While the prior art useseither a SOI substrate to control MEMS structure thickness or a hightemperature LPCVD process, the present invention sensor can befabricated on any substrate, using conventional mass productionprocesses for low cost consumer applications, due to the low processtemperature of the PECVD processes.

The conversion of mechanical strain to an electrical signal isaccomplished using a P-I-N or PIN diode integrated on a MEMS cantilever.If the P-I-N diode is fabricated on an amorphous film, the mechanicalstrain change on the amorphous film affects its current transportmechanism in the diode, resulting in a change in electrical current.Such a piezo effect is prominent and as a result the gauge factor (therelative current change per strain change) of an amorphous film P-I-N isas high as the gauge factor (the relative resistance change per strainchange) of a single crystal silicon piezoresistor. To take the advantageof this unique property of amorphous Si, a P-I-N diode junction can beused as a mechanical strain-to-electrical signal conversion transducer.

However, due to the high density of defects in amorphous silicon, thestrain across a large surface area is required to create current changeslarge enough to easily detect. The present invention piezo-diode MEMSsensor addresses this issue by using laser-crystallized silicon insteadof amorphous silicon to form P-I-N diode junctions. Laser-crystallizedsilicon has a better lattice quality, so carriers have a much highermobility. Therefore, a much smaller junction area is needed to generatedetectable changes in current due to surface stress.

Accordingly, a method is provided for fabricating a piezo thin-filmdiode (piezo-diode) cantilever microelectromechanical system (MEMS), themethod provides a substrate and deposits thin-films overlying thesubstrate. The substrate can be made of glass, polymer, quartz, metalfoil, Si, sapphire, ceramic, or compound semiconductor materials.Amorphous silicon (a-Si), polycrystalline Si (poly-Si), oxides, a-SiGe,poly-SiGe, metals, metal-containing compounds, nitrides, polymers,ceramic films, magnetic films, and compound semiconductor materials aresome examples of thin-film materials. A cantilever beam is formed fromthe thin-films, and a diode is embedded with the cantilever beam. Thediode is made from a thin-film shared in common with the cantileverbeam. The shared thin-film may be a film overlying a cantilever beam topsurface, a thin-film overlying a cantilever beam bottom surface, or athin-film embedded within the cantilever beam.

Subsequent to depositing the thin-films, in one aspect the thin-filmsare annealed by heating the substrate to a temperature exceeding 800° C.Then, subsequent to annealing, metal interconnects are formed to thediode P and N electrodes. In another aspect, the method forms a lateralPIN diode, having a serpentine pattern in a Si thin-film layer.

Additional details of the above-described method and an associatedpiezo-diode cantilever MEMS are provided below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial cross-sectional view of a piezo thin-film diode(piezo-diode) cantilever microelectromechanical system (MEMS).

FIGS. 2A through 2C are partial cross-sectional views depicting examplesof thin-film layers shared by the cantilever beam and diode.

FIG. 3 is a partial cross-sectional view of a piezo-diode cantileverbeam section made from two thin-film layers.

FIGS. 4A and 4B are partial cross-sectional views of a cantilever beamand diode sharing a common film layer.

FIGS. 5A and 5B are plan and partial cross-sectional views,respectively, of a piezo-diode cantilever MEMS formed form a lateral PINdiode.

FIGS. 6A through 6C depict partial cross-sectional and plan views forthree types of piezo P-I-N cantilever transducers.

FIGS. 7 through 19 depict steps in an exemplary piezo-diode cantileverMEMS fabrication process.

FIG. 20 is a flowchart illustrating a method for fabricating a piezothin-film diode (piezo-diode) cantilever MEMS.

DETAILED DESCRIPTION

FIG. 1 is a partial cross-sectional view of a piezo thin-film diode(piezo-diode) cantilever microelectromechanical system (MEMS). Thepiezo-diode cantilever 100 comprises a substrate 102. Some possiblesubstrate materials include glass, polymer, quartz, metal foil, Si,Si-containing materials, sapphire, ceramic, and compound semiconductormaterials, such as Si-germanium for example. A multi-layered filmcantilever beam 104 has a distal end 106 anchored to the substrate 102,and a proximal end 108. A diode 110 is embedded in the cantilever beam104 and shares a film layer in common with the cantilever beam 104. Thecantilever beam 104 has a top surface 112 and a bottom surface 114. Acavity 116 is formed between the cantilever beam proximal end 108 andthe substrate 102.

The cantilever body can be a rectangular as shown, or shaped (e.g., as atriangle, circle, or oval), with one or multiple points for anchoring tothe substrate. The cantilever beam 104 and diode 110 film layers may bemade from materials such as amorphous silicon (a-Si), polycrystalline Si(poly-Si), oxides, a-SiGe, poly-SiGe, metals, metal-containingcompounds, nitrides, polymers, Si-containing materials, ceramic films,magnetic films, compound semiconductor materials, or combinations of theabove-mentioned materials. The above-mentioned lists are not intended tobe an exhaustive list of every possible material, but rather someexamples of substrates and materials that can be deposited usingconventional thin-film deposition processes.

As used herein, a “thin-film” is defined as a film that is formed by adeposition process. Although the thin-films may be etched afterdeposition to create patterns, thin-film layers are often approximatelythe same as the original film deposition thickness. Thin-filmthicknesses vary according to the material and specific depositionprocess. Typically however, a thin-film has a thickness of less than 1micrometer. Some of the processes used to deposit thin-films includechemical vapor deposition (CVD), sputtering, evaporation, molecular beamepitaxy (MBE), and spin-coating.

FIGS. 2A through 2C are partial cross-sectional views depicting examplesof thin-film layers shared by the cantilever beam and diode. In FIG. 2A,the diode 110 is formed in a film layer 200 overlying the cantileverbeam top surface 112. In FIG. 2B, the diode 110 is formed in a filmlayer 202 overlying the cantilever beam bottom surface 114. In FIG. 2C,the diode 110 is formed in a film layer 204 embedded within thecantilever beam 104. Although not specifically shown, it should beunderstood that the diodes and cantilever beams made from multiplethin-film layers. Further, the diode and cantilever beam may sharemultiple thin-film layers.

FIG. 3 is a partial cross-sectional view of a piezo-diode cantileverbeam section made from two thin-film layers. Depending on the types ofmaterials used for example, a first film layer 300 may have a firststress level or stress condition, while a second film layer 302 has asecond stress level overlying the first layer 300. The differences instress between the two layers may be a design choice to maximize themovement of the cantilever beam under the influence of environment,biological influences, or physical forces. In some aspects, the two filmlayers may be the same material, but formed under different depositionconditions or exposed to different post-deposition processes. Althoughnot specifically shown, the cantilever beam may be formed with more thantwo film layers, where differences in stress between the multiple filmlayers contribute to the reactions of the cantilever beam toenvironmental influences.

The differences in stress between the first layer 300 and the secondlayer 302 may be managed to ensure that the cantilever bends “up”, asbending down may cause the cantilever beam to stick to the substrate.These built-in stresses act upon the cantilever diode regions and resultin electron mobility enhancement. This mobility enhancement is reflectedin the diode static characteristics, making the diode more sensitive toany longitudinal strain change caused by external forces, temperatures,chemical reactions and the like, acting upon the cantilever beam 104. Inother words, the built-in stress of the cantilever helps to enhance thestatic characteristics of diode via straining the active Si region toenhance the electron mobility. Any longitudinal strain change caused byexternal forces, temperatures, chemical reactions and the like, actingupon the cantilever beam 104 results in additional electron mobilitychanges, on top of the static electron mobility induced by the built-instress. In this manner, the stress change acting upon the active Siregion can be determined as a result of measuring changes in currentthrough the diode. Although not specifically shown, input and outputconductive traces are formed in one or more of the film layers, from thesubstrate to the diode, to communicate electrical signals and supply dcpower. The cantilever with diode converts external physical variablessuch as forces, velocities, accelerations, rotations, temperaturechanges, surface tension changes, photon input, and the like, toelectrical signals.

FIGS. 4A and 4B are partial cross-sectional views of a cantilever beamand diode sharing a common film layer. In FIG. 4A the cantilever beam104 is formed from a first film layer 400, made from a first materialand having a first crystalline structure 401 a. The diode 110 is formedfrom the first film layer 400 and has a second crystalline structure 401b different from the first crystalline structure 401 a. For example, thefirst film layer 400 may be Si, the first crystalline structure 401 amay be amorphous, and the second crystalline structure 401 b may bepolycrystalline. As shown, the first film layer 400 may overlie otherfilm layers. Film layers 402 and 404 are shown. Alternately but notshown, the first film layer may be embedded between other film layers,or underlie the cantilever beam bottom surface.

As used herein, a-Si and micro-crystalline Si are defined as a Si filmlayer having an average grain size of less than 0.1 micrometers.Polycrystalline Si is defined herein as having an average grain sizelarger than 0.1 micrometers. In one aspect, the diode is formed from atleast one Si active film layer, which may be either a-Si orpolycrystalline Si, CVD deposited at relative low temperatures andtreated by either a laser or low temperature annealing process. Sincethe cantilever and diode share the deposited active film, the diode neednot be fabricated on SOI or a single crystal substrate. Rather, thecantilever and diode are fabricated from thin-films that are subject toetching and/or surface micromachining techniques. Surface micromachiningfabricates the MEMS device using an additive process wherein successivelayers of sacrificial layers and thin-films are deposited on top of anunetched substrate. Surface micromachined devices can be fabricated onany substrate compatible with thin-film processing.

Because of the differences in crystalline structure, the cantilever beamfirst film layer has a first stress level, resulting from the firstcrystalline structure 401 a. Likewise, the diode first film layer has asecond stress level, different from the first stress level, as a resultof the second crystalline structure 401 b. Alternately stated, the filmlayer shared by the cantilever beam and diode may have differentmechanical characteristics, even though the film is made from the samematerial. For example, the first film 400 may been initially formed withan amorphous structure 401 a, and have been subject to a laser annealingprocess that formed a polycrystalline structure 401 b is the area to beoccupied by the diode 110. Such a process would improve electronmobility through the diode.

By selectively annealing the active regions and non-active cantileverregions, desired electrical and mechanical characteristics on differentregions can be managed in the same process step, by programming thelaser anneal system. Even if the cantilever is made from Si oxide, theoxide layers may be laser annealed, in the same or different processsteps than the annealing of the diode Si active layer, to altermechanical characteristics of the beam 104.

The annealing may occur is the same or separate annealing processes.Even if annealed in the same process step, two regions may be subject todifferent fluences, time durations, or number of laser shots. In thismanner, the grain size in the diode active region can selectively bemade larger than in adjoining regions. Also, this laser annealingprocess permits the mechanical characteristics of a film layer to bealtered independent of the rest of the materials on the substrate.

Alternately, different regions of the same film layer may have differentelectrical properties. That is, two regions in the same film layer neednot necessarily have a different crystalline structure, todifferentiated by electrical characteristics.

For example, in FIG. 4B region 402 b may be doped to enhance electronmobility, and so be suitable as a diode active layer. Thus, device 100may be explained as an integrated electromechanical stress sensor 100having a distal end 106 anchored to the substrate 102. The cantileverbeam 104 includes a plurality of film layers (e.g., 400, 402, 404)including a first film layer responsive to stress induced on a beamsurface 406, and a diode 110 formed in the film layer. Note; sincestress on the surface 406 is translated to underlying layers, the firstfilm layer need not necessarily be film layer 400. That is, thecantilever beam 104 includes a film layer (e.g., layer 402), with adiode 110 having an active region 402 b formed in the film layer.

In another aspect, the device of FIG. 4A or 4B may be seen as cantileverdiode 100. The diode 100 includes a cantilevered film layer (e.g., 400or 402) having a distal end 106 anchored to the substrate 102,responsive to environmentally-induced stress. The diode 100 alsoincludes an active Si region 401 b (FIG. 4 a) or 402 b (FIG. 4B) formedin the film layer 400 (FIG. 4A) or 402 (FIG. 4B).

Alternately, the device of FIG. 4A or 4B can be described as acantilevered microelectromechanical system (MEMS) diode 100. Thecantilevered MEMS diode 100 comprises a substrate 102, and a pluralityof cantilevered film layers (e.g. 400, 402, and 404) having a distal end106 anchored to the substrate. An active Si region 401 b (FIG. 4A) or402 b (FIG. 4B) is formed in one of the cantilevered film layers. Asshown, the active region is associated with diode 110. However, in othervariations the active region could be a transistor active layer.Although only a single active layer 401 b or 402 b is shown, as would tocase in a lateral diode or planar transistor, in other aspects notshown, multiple active layers may be stacked to form a verticalstructure. It should be understood that a “cantilevered film layer” isunderstood to be a film layer that is an integral part of the cantileverstructure, and not a film layer overlying a region of the substrateadjacent the cantilever, which may have been deposited simultaneouslywith a cantilevered film layer.

FIGS. 5A and 5B are plan and partial cross-sectional views,respectively, of a piezo-diode cantilever MEMS formed from a lateral PINdiode. The lateral PIN diode 110 has a serpentine pattern formed in a Sifilm layer 500 overlying the cantilever beam top surface 112, made froma p-doped region 502, n-doped region 504, and intrinsic region 506.Alternately but not shown, the lateral PIN diode may be formed on a Sifilm layer embedded with the cantilever beam 104, where embedding isdefined as covered with overlying and underlying film layers.

Functional Description

The present invention piezo-diode cantilever MEMS converts mechanicalenergy (surface stress) induced on a molecule-solid interface toelectrical energy. Reduced MEMS structure thicknesses can effectivelyenhance the mechanical strain induced by surface stress. To that end,the device is formed from thin-films deposited using a plasma enhancedchemical vapor deposition (PECVD) method for example, to control theMEMS structure thickness. Pre-deposited sacrificial films can be used tosupport the MEMS structure fabrication and define the air gap betweenthe MEMS structure and the substrate. The sacrificial film is removed tofree the MEMS structure after all fabrication steps are completed. Dueto the low process temperature of PECVD, the device can be integratedinto any mass production substrate for low cost consumer applications.

The conversion from mechanical strain to electrical signal isaccomplished using a P-I-N diode integrated on a MEMS cantilever. Themechanical strain change on the P-I-N diode active layer(s) affects thediode's current transport mechanism, resulting in the electrical currentchange. Due to the high density of defects in amorphous silicon,laser-crystallized silicon can be used to instead of amorphous siliconin the P-I-N junctions. Laser-crystallized silicon has better latticequality, so carriers have much high mobility, and the junction area canbe made smaller. Further, diodes made from laser-crystallized siliconfilms have less intrinsic noise and better detection resolution. Theoverall power consumption can be made small if the piezo P-I-N diode isreverse biased at low current. As described below, the diode can be madefrom lateral or planar junctions.

To integrate the present invention MEMS transducer on a low costsubstrate using low temperature processes, without sacrificingsensitivity, the transducer can be made from laser-crystallized siliconP-I-N junctions, and integrated on a low temperature PECVD TEOS silicondioxide cantilever, using PECVD amorphous silicon as the sacrificialfilm. At the end of the fabrication process, TMAH or XeF2 can be used toremove sacrificial film and free the cantilever from the underlyingsubstrate.

FIGS. 6A through 6C depict partial cross-sectional and plan views forthree types of piezo P-I-N cantilever transducers. FIG. 6A depicts avertical junction design, FIG. 6B depicts a lateral junction design,FIG. 6C depicts an “L” junction design. The vertical design has thelargest junction area and it is the most straight-forward design.However, separate deposition and doping steps are required to form theP, I and N regions. The lateral design has simplest structure, but thejunction area is limited. The “L” junction design is the tradeoffbetween the vertical junction design and the lateral junction design,offering greater flexibility in fabrication, to optimize transducerperformance.

Table 1 compares the differences in performance between the piezo P-I-Ndiode cantilever and a piezoresistor device formed on a single crystalSi substrate. Besides maintaining the same level sensitivity as singlecrystal silicon piezoresistor, the piezo P-I-N transducer consumes lesspower, has less 1/f noise, and is fully compatible with low temperatureIC fabrication processes.

TABLE 1 Piezo P-I-N vs. Piezoresistor comparisons Laser-crystallizedSingle crystal silicon silicon Piezo P-I-N Piezoresistor SensitivityHigh High Power consumption Low High Interface scattering Low LowTemperature sensitivity High High Carrier polarity Bipolar majoritycarrier Major noise mechanism Thermal Thermal and 1/f Process complexityLow Low Integration flexibility with low High Low temperature processDetection circuits requirements Low Low

FIGS. 7 through 19 depict steps in an exemplary piezo-diode cantileverMEMS fabrication process. In FIG. 7 the process starts with anysubstrate. TEOS is deposited to a thickness of 1500 Å using a PECVDprocess. Then, a 5000 Å a-Si mask is formed, and the a-Si mask ispatterned to obtain alignment marks. A PECVD process is used to deposittensile-stressed silicon oxide to a thickness of about 1500 Å. In FIG. 8a PECVD process deposits 1.5 micrometers (um) of a-Si. An RIE etch isperformed on the a-Si to define a sacrificial mesa in MEMS area. A maskis used to define dimples, time-etched into the a-Si mesa about 1 umdeep.

In FIG. 9 a PECVD process deposits 2500 Å of low stress TEOS SiO2 and500 Å of standard TEOS SiO2 as a “base coat”. In FIG. 10 a PECVD processdeposits 1000 Å of α-Si, which is laser-crystallized to formpolycrystalline Si.

In FIG. 11, a CVD process forms a 500 Å thick screen oxide. In FIG. 12 aphotoresist mask is used for P+ implantation. In FIG. 13 anotherphotoresist mask is formed for N+ implantation. In FIG. 14 an activationanneal is performed after the dopant implantation. In FIG. 15 contacthole openings are made, and in FIG. 16 silicide is formed in the openingto serve as electrical interconnections to the diode.

In FIG. 17 a sputter deposited layer (not explicitly shown) of Ti (100Å)/TiN (150 Å)/AlCu (8000 Å) forms bonding pads. A PECVD processdeposits 1000 Å of low stress TEOS SiO2 as passivation layer and MEMStop layer. In FIG. 18 an RIE process etches through all low stress TEOSlayers to form the outline of the MEMS structure. In FIG. 19 a wet etchis performed on the a-Si to release the cantilever from the underlyingsubstrate. In this example, a diode is also formed on the substrateadjacent the cantilever. For greater sensitivity, the cantilever diodeand substrate diode currents may be contrasted. Alternately, thesubstrate diode may be used to amplify the cantilever diode current.

Just as solid-state device designers first focus on the electricalproperties of any new film before attempting to build active devicesfrom that material (such as low-K dielectrics or copper interconnects)MEMS designers also focus on the mechanical properties of the filmsbefore constructing devices. The most important properties are the filmstress, stress gradient (e.g. how much the stress varies from the top ofthe film to the bottom of the film), stiffness, and behavior in variousetchants since at least one material is sacrificed to form a cavity,while other films survive.

The use of glass substrates offers unique opportunities to producesurface micromachined devices with low temperature processes at a muchlower cost.

TABLE 2 Comparison of Silicon vs. Glass substrates Silicon Attributesubstrate Glass substrate Cost moderate Low Max substrate size (m²)1 >2.7 Optical properties Transparent to Transparent to all IRwavelengths Electrical insulation poor Excellent Dielectric propertiespoor Excellent Biological compatibility poor Excellent Thermalinsulation poor Excellent Max temperature 1400 C. 650 C.Crystallographic bulk yes No etch

The optical transparency of glass (other than its obvious advantage fordisplays) permits the creation of novel MEMS devices. For example, it ispossible to optically sense the motion of a device through the substratewithout requiring through-holes or expensive packaging. MEMS devices canbe built on the same substrate as liquid crystal (LC) displays. Thisprovides opportunities to build other novel devices including a compactultrasound transducer integrated onto the same substrate as an LCDallowing for easy medical examination, a low-cost glucose monitor withintegrated LCD readout for diabetics, and a wide variety of othersensor+display elements. One of the stumbling blocks when developing RFand electromagnetic MEMS devices is the effect of the silicon substrate.Typically, large quantities of substrate must be removed to improve thequality of the MEMS device. By using a glass substrate, this process isnot necessary and the devices are simpler to manufacture and are morephysically robust (since the substrate is intact). Additionally, manyMEMS processes need to take special steps to electrically isolateindividual moving elements from each other when they're all attached tothe same conductive and parasitic substrate. Again, with glass, thisisolation is inherently not necessary.

Microfluidic and biological applications often require materials thatare bio-compatible, i.e. are biologically inert. Glass is one suchmaterial. It is simpler to start with a bio-compatible material (such asa glass substrate) than to use incompatible materials and coat them withappropriate surfaces.

Quite a few MEMS applications require thermal insulation betweenelements, such as bio-meters (IR sensors), field emission tips, andchemical detectors. With devices on a silicon substrate, much of thesubstrate must be removed to provide this thermal insulation. By using aglass substrate, each element is inherently isolated.

FIG. 20 is a flowchart illustrating a method for fabricating a piezothin-film diode (piezo-diode) cantilever MEMS. Although the method isdepicted as a sequence of numbered steps for clarity, the numbering doesnot necessarily dictate the order of the steps. It should be understoodthat some of these steps may be skipped, performed in parallel, orperformed without the requirement of maintaining a strict order ofsequence. The method starts at Step 2000.

Step 2002 provides a substrate from a material such as glass, polymer,quartz, metal foil, Si, Si-containing materials, sapphire, ceramic, orcompound semiconductor materials. Step 2004 deposits thin-filmsoverlying the substrate. Some examples of thin-film materials includea-Si, polycrystalline Si, oxides, a-SiGe, poly-SiGe, Si-containingmaterials, metals, metal-containing compounds, nitrides, polymers,ceramic films, magnetic films, and compound semiconductor materials.Step 2006 forms a cantilever beam from the thin-films. Step 2008 forms adiode embedded with the cantilever beam and made from a thin-film sharedin common with the cantilever beam. For example, the diode may be formedfrom a thin-film overlying a cantilever beam top surface, a thin-filmoverlying a cantilever beam bottom surface, or a thin-film embeddedwithin the cantilever beam.

In one aspect, forming the diode in Step 2008 includes forming a lateralPIN diode, having a serpentine pattern in a Si thin-film layer (see FIG.6 c). However, vertical and “L” shaped PIN diodes may also be formed.The present invention is not limited to any particular diodeconfiguration.

In another aspect Step 2005 a anneals the thin-films by heating thesubstrate to a temperature exceeding 800° C., subsequent to depositingthe thin-films in Step 2004 (see FIG. 14). Subsequent to annealing, Step2005 b forms metal interconnects to the diode P and N electrodes (seeFIG. 16). In a different aspect, Step 2005 c relieves stress in thethin-films in response to the annealing (Step 2005 a). Then, forming thecantilever beam in Step 2006 includes forming the cantilever beam in thestress-relieved thin-films.

In one variation, Step 2002 provides a glass substrate, sensitive totemperatures exceeding 600° C. Step 2010, subsequent to forming thediode, laser anneals selected regions of the thin-films. Then, Step 2012relieves stress in the thin-films in response to the laser annealing.

In another variation, Step 2005 a includes substeps. Step 2005 a 1 laseranneals a first region of the thin-films using a first annealingprocess. Step 2005 a 2 laser anneals a second region of the thin-filmsusing a second annealing process. In response to the first annealingprocess, Step 2005 c relieves stress in the first region of thethin-films. In response to the second annealing process, Step 2005 dforms a polycrystalline structure in the second region of thethin-films. As a result, Step 2008 forms the diode in the second region.

A piezo-diode cantilever MEMS and associated fabrication processes havebeen provided. Examples of various materials, dimensions, designs, andprocess flows have been given to help illustrate the invention. However,the invention is not limited to merely these examples. Other variationsand embodiments of the invention will occur to those skilled in the art.

1. A piezo thin-film diode (piezo-diode) cantilever microelectromechanical system (MEMS), the piezo-diode cantilever comprising: a substrate; a multi-layered film cantilever beam having a distal end anchored to the substrate, and a proximal end; a diode embedded in the cantilever beam and sharing a film layer in common with the cantilever beam; wherein the cantilever beam is formed from a first film layer, made from a first material and having a first crystalline structure; and, wherein the diode is formed from the first film layer and has a second crystalline structure different from the first crystalline structure.
 2. The piezo-diode cantilever of claim 1 wherein the cantilever beam has a top surface and a bottom surface; and wherein the diode is formed in a film layer selected from a group consisting of a film layer overlying the cantilever beam top surface, a film layer overlying the cantilever beam bottom surface, and a film layer embedded within the cantilever beam.
 3. The piezo-diode cantilever of claim 1 wherein the cantilever beam and diode film layers are made from materials selected from a group consisting of amorphous silicon (a-Si), polycrystalline Si (poly-Si), oxides, a-SiGe, poly-SiGe, Si-containing materials, metals, metal-containing compounds, nitrides, polymers, ceramic films, magnetic films, compound semiconductor materials, and combinations of the above-mentioned materials.
 4. The piezo-diode cantilever of claim 1 wherein the substrate is a material selected from a group consisting of glass, polymer, quartz, metal foil, Si, Si-containing materials, sapphire, ceramic, and compound semiconductor materials.
 5. The piezo-diode cantilever of claim 1 the cantilever beam includes: the first film layer with a first stress level; and a second film layer with a second stress level overlying the first layer.
 6. The piezo-diode cantilever of claim 1 wherein the cantilever beam first film layer is a Si-containing material having an amorphous crystalline structure; and, wherein the diode first film layer has a polycrystalline structure.
 7. The piezo-diode cantilever of claim 1 wherein the cantilever beam first film layer has a first stress level responsive to the first crystalline structure; and, wherein the diode first film layer has a second stress level different from the first level, responsive to the second crystalline structure.
 8. The piezo-diode of claim 1 wherein the diode is a lateral PIN diode, having a serpentine pattern formed in a Si film layer overlying a cantilever beam top surface.
 9. The piezo-diode cantilever of claim 1 wherein the cantilever beam includes: a first film layer first region with a first set of electrical characteristics; and a first film layer second region with a second set of electrical characteristics, different from the first set of electrical characteristics.
 10. A piezo thin-film diode (piezo-diode) cantilever microelectromechanical system (MEMS), the piezo-diode cantilever comprising: a substrate; a multi-layered film cantilever beam having a distal end anchored to the substrate, and a proximal end; a diode embedded in the cantilever beam and sharing a film layer in common with the cantilever beam; and, wherein the diode is a lateral PIN diode, having a serpentine pattern formed in a Si film layer overlying a cantilever beam top surface. 